Pidgin is one of the many multi-protocol instant messengers. And its available for many plattforms (Windows, GNU/Linux, Mac OS X, etc). Although pidgin supports many protocols out-of-the-box (f. ex. ICQ, AIM, IRC, XMPP/Jabber, Groupwise), new plugins are constantly emerging. The Pidgin-Plugin page gives a nice summary.
Some additional plugins I've come across are: